1. Field of the Invention
The present invention relates generally to the field of comminuting devices and more specifically to grinding stones useful therein. Still more specifically, the present invention relates to grinding stones which may be made at ambient temperature from inexpensive material and using readily available equipment. In its most preferred embodiment, the present invention relates to grinding stones useful in rubber comminuting devices and which are used in pairs, are shaped as a torus and which include molded grooves on the confronting surfaces of the stone pairs, and which, in an alternative embodiment, are formed in situ with a steel support plate.
2. Description of the Prior Art
Comminuting devices of various types are widely known for a variety of uses, such as the milling of grain and the grinding of rubber scrap into fine powders. The latter have a variety of end use applications, e.g. in asphalt paving mixtures and in roofing compositions. In many of the grinding devices, including those discussed below, a pair of stones having abrasive flat faces are brought into proximity and one or more of the typically annular stones is rotated about its axis to abrade, shred, disintegrate, or tear the starting material into small pieces. Some processes are cryogenic, i.e. conducted at cold temperatures. Others are conducted at room temperatures, while still others are conducted in slurry form. No matter what process is used or what the ground material may be, a common and costly problem is the useful life of the stones. Obviously, the harsher the application, the shorter the stone lifetime. In any event, in nearly all grinding operations, the cost of grinding stones and the downtime cause by the frequent repair or replacement thereof has a major financial impact on overall process profitability, efficiency, and in some cases even on process viability. The problems just discussed exist today in the rubber comminuting field where margins need to be carefully controlled and where grinding stone replacement and downtime costs represent a very substantial portion of the overall process economics.
A considerable amount of prior art describes grinding machines which can be used to grind material between abrasive grinding stones. Recently, the stones and plates on which they are mounted have been improved so that they may be rotated at higher speeds, thus permitting higher production rates and a much broader range of applications. Operational speed had previously been limited because of the low resistance of the stone to centrifugal and thermal stresses. Thermal stresses are induced when heat is generated by the grinding operation itself. For many applications, such as reducing rubber, plastic or wood material, abrasive grinding stones are preferable to metal discs which are both expensive and suffer additional disadvantages for these particular end use applications. Further background on early stone mounting techniques is contained in U.S. Pat. No. 4,841,623 issued to Rine on Jun. 27, 1989 and titled METHOD OF MOUNTING STONES IN DISC OR ATTRITION MILLS. The disclosed techniques include the use of molten sulfur, lead or other molding material deposited between a flange of the grinding wheel and the wheel itself, the wheel being slightly enlarged in diameter so that the molten material can hold it in place.
Another technique described in the aforementioned Rine patent uses a layer of specially processed material, usually rubber, to act as a cushion between the stone and its backing plate to relieve grinding strains and shocks. However, where heavy stress and torque loads are encountered, wire or other suitable binding is needed on the outside diameter of the stone.
Grinding wheels have also been found to be of greater benefit than cryogenic hammer-mill techniques which freeze elastomeric particles and crush them while they are in a frozen condition. Wet grinding is a process which has been developed and is described in early patents, such as British Patent No. 1,516,090 to Robinson, et al. and in a series of patents owned originally by The Goodyear Tire & Rubber Company of Akron, Ohio and exemplified by U.S. Pat. No. 4,469,284 issued to Brubaker et al. The method described therein uses abrasive stones, acting on a rubber particle/water slurry.
Most of the prior comminuting machines utilize vertical grinding machines in which a pair of opposed grinding stones are arranged with their horizontal surfaces facing each other. The top stone is typically fixed in place and the bottom stone is mounted on a motor arranged to rotate it about a vertical axis. Both stones have hollow centers and grind material between the mating faces of the stones which are formed like a flat torus. The material is introduced as a slurry through an opening in the top stone to an open center space formed between the center of the two stones. The slurry passes between the two opposing faces during the grinding process, and the ground slurry is collected in a collection region outside the outer rim of the stones. It is then processed by further steps of screening, drying and the like.
One of the present inventors has previously made several additional contributions to this art. For example, U.S. Pat. No. 5,238,194 issued to Rouse et al. on Aug. 24, 1993 titled METHOD OF PRODUCING FINE ELASTOMERIC PARTICLES describes a procedure in which the rate of flow of carrier liquid is established at a desired pressure when the stones are closed, thereby establishing a maximum flow rate of carrier liquid. The slurry is then fed at a flow rate equal to that established for the carrier fluid alone, whereby the gap between the two grinding stones will remain substantially constant and the production rate of ground material will be optimized.
Other techniques, including some of those described in various patents issued to Brubaker, involve the use of hydraulic means to set the spacing between the stones, while other companies have used mechanical devices to set the stone spacing.
Another contribution made by the assignee of the present invention is two-stage grinding, in which finer particles are produced by a two step process. See U.S. Pat. No. 5,411,215 issued May 2, 1995 to Rouse titled TWO STAGE GRINDING. The feed from the feed stock is first transported through stones to produce a -30 to -40 mesh product in a single pass, and water is then added to the resultant product and transported back through a second grinding stage to produce an average of -80 mesh powder.
A still further advance in this art is disclosed in U.S. Pat. No. 5,564,634 issued on Oct. 15, 1996 to Rouse et al. titled RUBBER COMMINUTING APPARATUS. In this device, instead of using stones mounted for rotation about a vertical axis, rotation around a horizontal axis is employed. Furthermore, the device described in the preferred embodiment of this patent includes two vertically mounted grinding stones which do not rotate and which are spaced apart from one another to act as stators. Located between them is a pair of outwardly facing rotor stones. The spacing between the opposed sets of grinding stones is established by increasing or decreasing the spacing between the stators, and while the rotors rotate for grinding, the floating center stones will position themselves equally between the two stators so as to equalize the dynamic slurry pressure imposed during the grinding process upon the faces of the stone.
The particular grinding stones used in such processes have also been described in the aforementioned '634 patent. The stones themselves have various grit sizes established by known techniques, and the mounting of them to the various support plates is also described in this patent. A preferred mounting technique is that described in the aforementioned Rine patent where grinding discs are placed under a compressive load sufficient to counter the tension loads during use. Preferably the compression loading is provided by taper elements incorporating the wheels themselves or by taper elements other than the wheels, such as fluid actuated clamps and elements external to the wheels that induce the compression. Examples of each are shown in FIGS. 1 and 2 of the Rine patent. The Rouse et al. '634 patent also describes an inherent disadvantage in the Rine system. Namely, because of the clamping structure used to counter the tension loads, the stones may only be used until they are worn down to the upper surface of the clamping members. At this point, the stones must be replaced. Adhesive mounting of stones, permitting the stone to be ground essentially down to the level of the stator can nearly double the life of the stone. While adhesive binding of the stones to the mounting plates is referred to in this patent, no examples are provided of suitable systems, and, to the knowledge of the present inventor, no suitable commercial systems based upon the use of adhesives alone to bind the stone to the backing plates are in commercial operation.
Grinding stones used in the aforementioned systems typically include silicon carbide as the grit material and a vitrified ceramic binder. Such stones are expensive, especially in situations where only a portion of the stone thickness is utilized (e.g., in systems where a mechanical stone containment system) such as that described in the Rine '623 patent, is used. The cost of the stones is due in large part to the cost of stone ingredients and to the cost of the energy required to melt the ceramic binder and to apply pressure to the stones during their molding processes.
Less expensive grinding stones, especially ones which retain grinding properties similar to those currently in use, would represent a very significant advance in the particle comminuting art.